Protection of metals



Patented Aug. 28, 1934 1,971,761 a PROTECTION OF METALS William J.Travers, Buffalo, N. Y.

No Drawing. Application September 5, 1929,

, Serial No. 390.651

12 Claims. (01. 204-8) Y This invention relates to the protection ofmetals and more particularly to an improved method of plating aluminum,its alloys and similar metals or alloys.

Aluminum and its alloys have at the present time an extensive use in'thearts. Thesematerials possess physical characteristics such as lightweight and relative cheapness which recommend their use in variousfields. These materials, however, do possess some characteristics whichrestrict their utility. For example, there is a large demand for metalswhich combine the advantages of relatively low weight with high surfacewearing qualities. This fact has been recognized before and manyattempts have been made in the past to provide such a. compositestructure by coating aluminum with a relatively non-oxidizable metalsuch, for example, as nickel. It is to this type ofprocess that thepresent invention re- 39 lates.

Within recent years the use of aluminum, through its incorporation inthe high strength alloys of the duralumin type, has become greatlyextended, particularly in the automotive and aeronautical fields. Thereis a continually in= creasing use of duralumin not only in therelatively massive forms but also in the sheet type. A disadvantage ofthe duralumin sheet when used for example upon airplanes, is that it issubjected not only to surface oxidation but to intercrystallinecorrosion. Because of the greatly increasing use of sheet duralumin, thechange in tensile properties resulting from intercrystalline corrosionbecomes a matter of great importance. Various means have been suggestedfor pre= venting the deterioration of duralumin resulting iromintercrystalline corrosion. These may be considered under two groups.The first group of treatment comprises a special heat treatment ofduralumln to increase its intercrystalline corrosion resistance. Thesecond group comprises protecting the surface against the attack of corrosive agents by a protective coating. These coatings generally consistof surfaces formed by anodic oxidation or by coating the sheet with animpervious resinous varnish or a bituminous paint. v

The present invention therefore contemplates the treatment not only ofcommercial aluminum but also of the high strength wrought alloys of theduralumin type, and other similar high strength light weight alloys.

As indicated above, attempts have been made' in the past to protectaluminum by electro-depositing a coat of another metal. Theelectro-deposition of a metallic coat upon aluminum presents peculiardifilculties. Deposited coats of nickel upon aluminum appear to bepeculiarly susceptible to peeling and stripping. This would appear to bedue largely to two main factors. The first is the formation of a film ofoxide upon the aluminum base, and the second, the peculiar unctuous orgreasy characteristic of aluminum metal.

Considered generally, the prior attempts at plating of similar metalsupon aluminum consist in the utilization of an intermediate metalliclayer. As a general rule, this intermediate deposit is of a metal of arelatively low fusion point. The rationale of such a use appears to bethat the interposed coat of the third metal will either fuse atrelatively low temperatures and thus serve as a bonding agent or will infact form a true alloy with the aluminum. Another method comprises firstsubjecting the aluminum base metal to a. sort of corrosion action sothat the surface to be plated is etched and thus forms a better adherentsurface for the subsequent electroplated metal.

It is a major object of the invention to provide 39 a method ofdepositing a relatively non-corrodible metal as a firmly adherentcoating upon an aluminous foundation metal.

Another object is to provide a method of plating light weight highstrength alloys with a metallic surface of high wearing qualities.

A further object is to provide a method for plating nickel on aluminousfoundation metals as a smooth and impervious surface.

A further object is to provide a structural material which comprises alight weight high strength foundation metal bonded or cemented toanother metal of greater resistance to corrosion and greater hardness.

With these and other objects in view the invention comprehends a specialtreatment of aluminum, aluminum alloys and similar alloys followed byart electro-deposition of a metallic coat so as to provide a very firm,adherent, continuous and impervious surface.

In order to explain the underlying principles of the invention, severalspecific treatments will be described hereinafter. It will beunderstood, however, that these are given by way of example and asexplanatory of the principles herein involved rather than as exclusivemethods of carrying out the invention.

In carrying out the process, the object to be treated, whethercommercially pure aluminum or a high strength alloy of the duralumintype,

, a marked white appearance.

the aluminum surface.

is first cleaned. If a very smooth deposit is required on the finishedarticle, it is advisable first to polish the article to be plated. Inany case the initial step in the process is degreasing. As to this partof the process, I have found that the ordinary alkaline solutions have ahigh corrosive action upon the aluminum base metal and should beavoided. For this reason I use a special type of cleaning or degreasingsolution.

This special solution comprises an alkaline solution such as sodiumcarbonate, sodium phosphate or other alkaline degreasing solutions towhich is added a soluble silicate such as water glass. I find that thissolution very effectively removes the grease and has little, if any,dissolving action on the aluminum. While I do not intend to berestricted to any statement as to the mechanics of this process, I am ofthe opinion that water glass acts in the nature of an inhibitor, for Ihave found that with the addition of this adjuvant the evolution of gasbubbles is greatly decreased. This appears to be borne out by the factthat other substances having an inhibiting action, such for example asarsenic trioxide, may be employed in lieu of the sodium silicate. I havefound a solution consisting of six to eight ounces of trisodiumphosphate with two to four ounces of a water glass solution in a gallonof water to be very effective. In use this solution should be heated to160 F. more or less. In using such a solution, it is advisable toreplenish the chief detergent constituent, namely 'the trisodiumphosphate, from time to time, to maintain the effective concentration.

While I prefer to use the degreasing solution described above, it is tobe clearly understood that the invention is not limited thereto. Othermeans of removing grease, as for example utilizing mechanical detergentssuch as fine pumice or organic solvents such as carbon tetrachloride,

'may be employed. However, as indicated, the

degreasing solution described above I prefer because the final resultsare in some degree infiuenced by the efiicacy of the cleaning ordegreasing treatment.

After the article has been degreased or mechanically cleaned in themanner described, it 'is then washed and cleaned chemically. Thischemical cleaning preferably comprises immersing the article in asolution of hydrofluoric or nitric acid for a brief period. An acidstrength should be chosen which will but slightly attack As an exampleof this acid dip, there may be employed a solution containing betweenone and two ounces of 50% hydrofluoric acid in a gallon of water. I havefound that improved results are obtained if the chemical activity of thesolution is restricted by the additions of small amounts of an inhibitorsuch, for example, as hydrogen peroxide. In a typical acid bath, such asthat given above, ap-

proximately two ounces of strong hydrogen peroxide to the gallon issufficient. The quantity of this required may readily be determined bynoting the dimunition in the evolution of gas bubbles on the aluminum.The article is retained immersed in the solution until it assumes Thissurface appearance sets up in a few seconds and is readily discernible.After this treatment, the article 'is then removed and rinsed. Duringoperation the hydrogen peroxide will be slowly reduced and hence itshould be replenished from time to time when the activity of the acidbecomes too great.

As intimated above, instead of I the hydrofiuoric acid, other acids suchas nitric acid may be employed. A typical nitric acid pickle may consistof a 30% solution of commercial acid. The concentration of the acidemployed, of course, should be carefully regulated so as to preventaccelerating action on the article undergoing treatment.

After the preliminary steps of degreasing and cleaning, the article isthen subjected to a special and novel treatment. This consists of an, soto speak, anodic oxidation treatment of the surface of the metal to beplated. It has been found recently that the intercrystalline corrosionof high strength aluminum alloys may be greatly diminished or lessenedby subjecting the material to anodic oxidation. However, the purpose ofthis is to form on the finished article an oxide coat. The aluminumoxide thereby formed presents a protective layer or coating for thesubjacent material, This is to be carefully distinguished from thepresent invention wherein an electrolytic treatment is employed toprepare an aluminous or similar base alloy to receive a plate of metal.This present process is in sharp contradistinction to prior artprocesses, for in all of these every atempt was made to remove existingoxide coatings and toprevent the formation of any additional oxides onthe surface to be plated.

While this treatment is described as anodic oxidation, it may not infact be, strictly speaking, an oxidation process and it is to be clearlyunderstood that I do not wish to be limited to any statement of reactionoccurrence. It would seem, by analogy to anodic oxidation processes nowemployed on duralumin, that such an action did obtain. In any event thepresent process is based on the discovery that if an aluminum surface issubjected to treatment to be described hereinafter, the condition of thesurface of the material is so modified that it becomes capable ofreceiving a smooth, highly adherent and practically impervious coatingof an eleotro-plated metal.

Briefly stated, the electrolytic treatment comprises immersing thearticle to be plated, as the anode, in an electrolytic cell. In carryingout the step a number of electrolytes may be employed. These may beconsidered generically as comprehending two classes, namely electrolytescontaining halogen compounds and those containing non-halogen compounds.Of these in the first group, I have found that excellent results may beobtained by employing electrolytes containing chlorine. A typicalexample comprises a solution containing ammonium chloride as the basicingredient. In utilizing this electrolyte, improved results are obtainedby acidifying a relatively weak acid such as oxalic or phosphoric acid.When the solution has been acidified, it has been found that theelectrolyte maintains its activity and clarity over longer periods oftime.

However, the acidification of the solution is not essential assatisfactory results have been obtained with alkaline solutions. Torender the solution alkaline, ammonium hydroxide or a similar base maybe added.

While ammonium chloride has been described as the prefered basicingredient of the electrolyte, it is to be understood that this ismerely typical; other halogen-containing compounds such as sodiumchloride, sodium bromide, and so forth may be employed. This class Ihave characterized as electrolytes containing halogen derivatives.

Jib

1,971,761 As noted above, other non-halogen electrolytes may beemployed. Typical examples of these are nitric acid, phosphoric acid,ammonium hydroxide and so forth. In using this type of electrolyte, ithas been found desirable to utilize an alternating current rather than adirect current, which latter may be employed with the first type ofelectrolytes. In lieu of the alternating current a reversing directcurrent may be used.

The electrolytic cell, in which the treatment is carried out, mayconsist of a container provided with suitable cathodes, such as graphiteelectrodes. These electrodes are preferably disposed on opposite sidesof the electrolytic bath and the article to be treated passedtherebet'ween. As noted above, the aluminum article is connected to thepositive side of the current supply. In operation the ordinary 6 to 12volt plating current may generally be employed. In the ordinarytreatment an electric current is passed through the electrolyte forseveral minutes at a current density of 25 amperes per sq. ft. more orless. It will be understood that the current density may be varied overa relatively wide range by allowing greater or less time for thetreatment.

After such an electrolytic treatment, the aluminum article will usuallyassume a darkened appearance, the depth of color depending to a largeextent upon metal treated. After such a treatment for a requisite periodof time, the article may be removed and after rinsing may be plated.

However, I have found that improved results are obtained if, prior tothe plating treatment, the treated article is subjected to a final dip.Preferably this consists in immersing the article in a relatively weaksolution of nitric acid. The composition of this solution may beapproximately thirty fluid ounces of 36 nitric acid to a gallon ofwater. This acid solution may be used either cold or heated. I havefound also that a saturated solution of oxalic acid may be employed inlieu of the nitric acid, and that bane-- ficial results are likewiseobtained.

I have found in numerous treatments, according to the above disclosure,that when the electrolyte consists of a solution of phosphoric acid orammonium hydroxide, etc., it is preferable to use analternating current.If a low voltage direct current is employed, an oxidized film is rapidlyformed upon the aluminum surface which retards or prevents furtheraction. However, by using alternating currents the desired anodic filmis formed. In using the alternating current method the A. C. amperageshould be maintained between about 10 to 20 amperes per sq. ft. When thearticle is subjected to the action of an alternating current in the acidelectrolyte, it has been found that it should not be placed directly inthe plating bath, as may be done when electrolytes of the halogen typeare employed. Articles treated in the non-halogen electrolyte shouldfirst be given a dip in the nitric or oxalic acid solution describedabove.

It is found that as a general rule a cold nitric or oxalic acid dip ispreferable for the aluminum alloys. A final dip in the acid solution isof a brief period and the article should not be maintained immerseduntil gas bubbles are evolved,

for it is found that with prolonged immersion the proper surfacecondition, attained by the electrolytic treatment, is detrimentallyaltered.

It appears that when the electrolyte contains an element of the halogengroup the electrolytic action, that is the action produced by theelecthe nature and purity of thetrlc current, is probably modifiedsomewhat by the simultaneous action of the halogen constituent. When anelectrolyte of the non-halogen type, such as a phosphoric acid solutionis employed, this modification factor is not present and hence itbecomes necessary to subject the material to subsequent treatment, thatis to the final acid dip in order to introduce this modification. Thismodification of the surface of the material, achieved either by thesimultaneous action of the halogen compound or by a subsequent dip, isdifficult of explanation. It appears. however, that this action is ineffect a partial removal of the anodic film.

After the treatment hereinbefore described, the article may then beplated with the desired metal. As a typical example, the article may beintroduced into one of the well known nickeliferous baths such as nickelammonia sulphate acidulated with boric or acetic acid. However, it willbe appreciated that other plating baths and other plated metals may beemployed. Nickel is described as a typical example of metals of thisgroup because it isone of the best metals for plating on aluminum.-;v

It is found that the coat of metal deposited on the aluminum object,after the described treatment, is highly adherent, smooth, continuous orsubstantially impervious. Improved adherence qualities of the plate areundoubtedly due to the preliminary treatment and more particularly tothe electrolytic action discussed.

It will be appreciated, therefore, that this treatment may be employedwhen the aluminum article is to be submitted to other specific platingoperations followed by mechanical working.

As an example, the following process is contemplated. It is known, ofcourse, that the improved tensile characteristics of duralumin are asmuch a function of the heat treatment as of the constituents of thealloy itself. This heat treatment of duralumin usually consists inheating an alloy up to a certain definite temperature and then rapidlyquenching. After the quench, the alloy may be aged at room temperatureor subjected to accelerating aging conditions, that is to say aging atelevated temperatures. Furthermore it has been found that the aging ofduralumin, that is the increase in tensile strength, does not beginuntil some time after the quenching. It is furthermore known that thetensile properties of this and similar alloys may be improved byworking.

The advantages of the maximum tensile strength of duralumin and thelustrous and relatively non-corrodible surface of nickel may be obtainedby utilizing the principles of the present invention. Instead of usingthe typical nickel plating bath, the nickel may be deposited on thealuminum base metal or alloy as an annealible and ductile metal. Forexample, the nickel may be deposited from a bath comprising nickelchloride, nickel sulphate and boric acid.

In lieu of the above, other plating baths from which nickel is depositedas a ductile metal may be employed. The aluminum alloy treated in thismanner may be an alloyof the duralumin type which has-been heated andquenched but alloy may be heated up to the proper temperature plate andthe foundation metal.

ceptible to materials, plated 4 in a fusednitrate bath and thenquenched. It is to be noted here that heating should be done underreducing conditions because of the well known afiinity of nickel foroxygen. After I quenching, and before the foundation metal betively widevariety of specific treatments of a' plated aluminum base metal whichmay be carried out. It will be appreciated that any of the according tothe descriptions, may be overplated with another metal, such aschromium, cadmium and so forth, by well known methods. The choice of theover-plated metal will, of course, depend upon the particular use forwhich the article is intended.

While several specific processes and compounds have been described asembodying the principle of the invention, it is to be understood thatthese are given merely as examples, for since these principles may beincorporated in other specific processes, utilizing different materials,I do not intend to be limited to those described except as suchlimitations are clearly imposed by the appended claims.

Iclaim:

1. A method of plating aluminumand aluminum-containing articlescomprising passing an electrical current through an electrolyte in whichthe article is immersed as an electrode to, form an m anodic film on thesurface of the article, subsequently subjecting the article to theaction of a solution of an acid to partially remove the anodic film, andthen electro-depositing a metal on the treated'surface of the article.

2. A method of plating aluminum and aluminum-containing articlescomprising passing an electrical current through an electrolyte in whichthe article is immersed as an electrode to form an anodic film on thesurface of the article, subsequently subjecting the article to theaction of a solution of nitric acid to partially remove the anodic film,and then electro-depositing a metal on the treated surface of thearticle.

3. A method of plating aluminum and aluminum-containing articlescomprising passing an electrical current through an electrolyte in whichthe article is immersed as an electrode to form an anodic film on thesurface of the article, subsequently subjecting the article to theaction of a solution of oxalic acid to partially remove the anodic film,and then electro-depositing a metal on the treated surface of thearticle.

4. A method of plating aluminum and aluminum-containing articlescomprising passing an electrical current through an electrolyte in which5 the article is immersed as an electrode to form an anodic film on thesurface of the article, subsequently subjecting the article to theaction of a solution of a reagent so selected and for a time sufficientto only partially remove the anodic film, and then electro-depositing ametal on the treated surface of the article. r

5. A method of plating aluminum and aluminum-containing articlescomprising passing an electrical current alternately in oppositedirections through an electrolyte in which the article is immersed as anelectrode to form an anodic film on the surface of the article,subsequently subjecting the article to the action of a solution of areagent so selectedand for a time sufficient to only partially removethe anodic film, and then electro-depositing a metal on the treatedsurface of the article.

6. A method of preparing aluminum and aluminum-containing articles forthe reception of an electrically deposited metal comprising passing anelectrical current through an electrolyte in which the article isimmersed as an electrode to form an anodic film onthe surface of thearticle, subsequently subjecting the article to the action of a solutionof a reagent so selected and for a time sufilcient to only partiallyremove the anodic film.

7. A method of electro-plating articles formed of aluminum alloys of theduralumin type comprisin'gpassing an electrical current through anelectrolyte in which the article is immersed as an electrode to form ananodic film on the surface of the article, subsequently subjecting thearticle to the action of a solution of a reagent so selected and for atime sufiicient to only partially remove the anodic film, and thenelectro-depositing duc- =tile nickel on the surface thereof, and thensubjecting the article to accelerated aging supplemented by coldworking.

8. A metallic article formed of aluminous material having a metalelectro-deposited on the surface thereof, which article prior to theelctrical deposition of metal thereon has been treated by passing anelectric current through an electrolyte inwhich the article is immersedas an electrode to form an anodic film on the surface of the' article,and by subjecting the so treated article to the action of a solution ofa reagent so selected and for a time sufficient to only partially removethe anodic film.

9. A method of plating aluminous material, comprising utilizing thematerial as an electrode, immersing'the electrode inan electrolyte,subjecting the electrode to an alternating current so as to form ananodic film on the surface of the material;"then subsequently subjectingthe material to the action of a solution of an organic acid to alter andpartially remove the formed film, and then electro-depositing a metal onthe treated surface.

10. A method of plating aluminous articles, comprising passing analternating current through an electrolyte containing oxalic acid, andin which the aluminum article is immersed as an electrode, forming afilm on the aluminum surface, then subjecting the so-treated article tothe action of a hot solution of tartaric acid to alter the film, andthen electro-depositing the material on the so-prepared surface.

11. A method of preparing cast aluminum articles, which comprisesutilizing the aluminum article as an anode in an electrolyte containingnitric acid, then subjecting the so-treated surface for a short periodof time to the action of a hot solution of nitric acid, and thenelectro-depositing a differential metal on the surface.

12. A method of preparing aluminum articles of the casting type forelectro-plating, which comprises subjecting the aluminum article to ananodic treatment in nitric acid, removing the article from theelectrolyte and subjecting it to an acid dip and then electro-depositinganother metal on the treated article.

, WILLIAM J TRAVERS.

an electrolyte containing I

